Conventional methods of manufacturing and purifying non-chlorine-containing fluorocarbons typically result in a product containing at least a small amount of undesired impurities. These fluorocarbons are useful as refrigerants, blowing agents, cleaning agents and many other applications. When any of these compounds are used as an etchant in electronics applications, purity requirements are unusually high. Even trace impurities can markedly affect the reject rate for miniaturized electronic circuits or optical disks, in some cases affecting the reject rate a thousand-fold. As a result, highly purified fluorocarbons used for such applications typically require unusually stringent purification procedures and command a premium market position and price. Two of the compounds so used are trifluoromethane (HFC-23) and hexafluoroethane (FC-116).
It is known that HFC-23 can be made in a mixture with chlorofluoromethanes by intensive fluorination of chloromethane compounds under special conditions, or by disproportionation of chlorodifluoromethane (HCFC-22). These reactions are costly to carry out on a small scale because of the small volume of HFC-23 required, and the reaction products require extensive purification to obtain a product of sufficient purity for the etchant market.
It is also known that commercial processes to make the important refrigerant chlorodifluoromethane (HCFC-22) on a large scale by catalytic reaction of chloroform with HF typically also produce several percent of HFC-23 as a byproduct. Depending on the size and design of the plant, this HFC-23 can be vented to the atmosphere or recovered. The environmentally desirable recovery of HFC-23 for sale to the etchant market is costly because of the small percentage produced and the number of impurities which must be removed.
It is also known that FC-116 can be made by similar catalytic processes involving the reaction of HF or fluorine with perchloroethylene or other two-carbon halocarbons. Again, these processes yield a product that requires extensive purification for the etchant market.
Other fluorocarbons such as perfluoroepoxides are useful in preparation of various fluoropolymers. One of the most important perfluoroepoxides is hexafluoropropylene oxide, or HFPO, which is used to make a variety of specialty fluorochemical polymers with complex structures. The presence of small amounts of impurities interferes with many of the subsequent processing steps, particularly in polymerization, where low levels of impurities can have a serious limiting effect on achieving the desired molecular weight polymer. Hexafluoropropylene oxide (HFPO) is typically manufactured by oxidation of hexafluoropropylene (HFP) using oxidizing agents such as hydrogen peroxide, sodium hypochlorite, oxygen or ozone.
Most of the bulk impurities from the above reactions to make HFC-23, FC-116 or HFPO can be readily removed from the desired fluorocarbon by careful fractional distillation and/or scrubbing to remove acids, followed by drying by passing through a silica gel bed. When HFPO is manufactured by oxidation of HFP, HFP can be present in the stream to be purified but would usually not be considered to be an impurity. However, even after careful purification, these compounds typically contain small amounts of carbon dioxide (CO.sub.2). This may result from the presence of CO.sub.2 in the water used for scrubbing acidic impurities, as a byproduct of the reaction, or from other sources. For generally non-reactive fluorocarbons such as HFC-23 and FC-116, the amount of CO.sub.2 can be reduced by scrubbing the fluorocarbon with an excess of caustic solution (relative to the CO.sub.2), or by passing it through a fixed bed of soda-lime pellets, also present in excess relative to the amount of CO.sub.2. The reactions involved are shown below: EQU 2 NaOH+CO.sub.2 .fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 O EQU Ca(OH).sub.2 +CO.sub.2 .fwdarw.CaCO.sub.3 +H.sub.2 O
However it is difficult to achieve reliably low levels of CO.sub.2 with either of these approaches because the needed excess of alkali results in an alkali-alkali carbonate mixture, the composition of which must be carefully and continually monitored for maximum effective removal of the CO.sub.2. That is, if the proportion of alkali carbonate in the resulting alkali-alkali carbonate mixture becomes too high, the mixture becomes less effective in removing CO.sub.2, and the product no longer meets specifications [a current goal is 50 parts per million (ppm) of CO.sub.2 on a molecular or volume basis, with a future goal of 10 ppm]. If the alkali-alkali carbonate mixture is replaced with fresh alkali while the proportion of alkali carbonate is too low, the cost of the operation becomes excessive. In addition, either approach creates an alkali-alkali carbonate mixture which must be disposed of. Furthermore, either of these steps introduces some water (from the scrubbing solution and/or as neutralization byproduct) into the dry fluorocarbon which must then be removed in an additional step.
For more reactive fluorocarbons such as HFPO, scrubbing with an alkali may give rise to unwanted side reactions and yield losses.
Processes have been proposed for removing trace quantities of impurities from etchant gases by contacting them at high temperatures with special Zr-V-Fe alloys as disclosed in EP 501 933 A2, or by contacting with hydrogenated Ni-NiO catalysts as disclosed in JP 06116180 A2, in order to react with and remove the impurities. These methods of treatment are costly.
It is also known to carry out polymerizations of fluorinated monomers in media comprising CO.sub.2. See, for example, U.S. Pat. No. 5,674,957. Unreacted monomers from such processes are desirably recovered from mixtures with CO.sub.2 for recycle to the polymerization reaction.
There is a need for a process to remove low or trace quantities of CO.sub.2 from fluorocarbons such as HFC-23, FC-116 or HFPO in a reliable manner without contacting them with other chemicals which can introduce water or other impurities and create waste disposal problems or problems in polymerization for polymerizable monomers, or require costly alloy or catalyst reaction treatments.